When Stephen Timms, the Labour MP, was stabbed last year during a constituency surgery, no one suggested that it was "caused" by the polarisation of politics in this country. Similarly, when the Liberal Democrat MP Nigel Jones was attacked, and his assistant Andrew Pennington killed, by a sword-wielding constituent a decade ago, blame was attached to the assailant, not to the state of political discourse.

Yet Saturday's attempted murder of Gabrielle Giffords, the Arizona congresswoman, and the shooting dead of six onlookers by a deranged gunman has triggered a fierce debate in the United States about the health of its democracy. More specifically, many liberals are blaming the raw and aggressive rhetoric of supporters of the Tea Party movement for creating the climate in which such a dreadful crime could be committed. This, perhaps, is the kind of language they have in mind: "If they bring a knife to the fight, we bring a gun. Because from what I understand, folks in Philly like a good brawl." This is, indeed, incendiary talk – and was uttered by Barack Obama on the campaign trail in Philadelphia in 2008.

This shows the danger – as well as the crass tastelessness – of using such horrific events as the Tucson shooting to try to score cheap political points. The alleged killer appears to have been a mentally deranged misfit acting alone, yet Mr Obama's supporters wasted no time in using the act to vilify their political opponents. The American political classes would do better to focus on gun-control laws that allow, in Arizona, a 22-year-old with mental health problems to buy a Glock 19 handgun. But with the President (and indeed Miss Gifford) staunch defenders of the individual's Second Amendment right to bear arms, such a debate is unlikely to get very far.